Ctenophora, Platyhelminthes & Aschelminthes (Nematoda)

Ctenophora, Platyhelminthes & Aschelminthes (Nematoda)

Phylum-Ctenophora or Coelenterata- Ctenophora is a small phylum.

- It contains only about 80 species.

- It includes a set of marine animals commonly called comb jellies or sea walnuts.

- These animals exhibit the characters of Coelenterata and platyhelminthes.

- Formerly this phylum was placed under Coelenterata. Hatschek (1889) placed it under a separate phylum called Ctenophora.

- The important Ctenophore animals are Pleurobachia, Coeloplana, Ctenoplana, Velamen, hemiphora, Beroe, etc.

Salient Features

Phylum Ctenophora shows the following salient features

(1) All the ctenophores are marine.

(2) They are solitary and pelagic.

(3) They are transparent.

(4) They have tissue-grade of organization.Structure of Ctenophora

(5) They have biradial symmetry.

(6) They are acoelomate animals.

(7) They are unsegmented.

(8) They body-wall is diploblastic

(9) The mesogloea contains cells.

Anal Pores

(10) Nematocysts are absent.

(11) Special adhesive cells called colloblasts are present in all ctenophores.

(12) The gastrovascular system is well developed.

(13) Two anal openings are present.

(14) Skeletal system is absent.

(15) Excretion and respiration are carried out by diffusion.

(16) The nervous system is in the form of nerve net.

(17) An aboral sense organ in present in the form of statocyst.

(18) Cilia are used for locomation.

(19) They are hermaphrodites.

(20) Development is indirect. It includes a cydippid larva.

Classification of Ctenophora

Class 1. TentaculataClass Tentaculata belongs to Phylum-Ctenophora

(1) The body is simple, rounded or oval or ribbon-like.

(2) Two long aboral tentacles are present.

(3) Mouth is narrow and pharynx is small.

Order 1. Cydippida

(1) Body is oval or rounded

(2) Two long branched tentacles are present and they can be retracted into sheaths

(3) Branches of gastro–vascular system are terminating blindly

(4) Examples : Hormiphora, Pleurobrachila, etc.,

Order 2. Lobata

(1) Body is laterally compressed

(2) Two large oral lobes or lappets and four pointed processes or auricles are presen

(3) Tentacles are many, non–retractile without sheaths.

(4) Stomodaeal and meridional vessels unite with one another

(5) Examples : Deiopea, Bolinopsis, etc.,

Order 3. Cestida

(1) Body is laterally compressed and ribbon-like

(2) Two main tentacles and may lateral tentacles are present

(3) Four rows of rudimentary comb plates are present

(4) Meridonal and stomodaeal vessels anastomose

(5) Examples : Cestum, Velamen, etc.,

Order 4. Platyctenea

(1) Body is worm-like and compressed in oral-aboral axis

(2) Tentacles with sheaths are present

(3) Comb rows or swimming plates are present only in larva

(4) Meridional canals are absent, but there is a system of branching peripheral system

Class 2. Nuda

(1) Body is large thimble-shaped or conical

(2) Tentacles are absent

(3) Mouth is wide and pharynx is large

(4) The meridional vessels are produced into a complex system of anastomosing branches

(5) Example : Beroe

Question 1: The phylum of comb jelly is

(a)   Mollusca                       

(b)   Echinodermata          

(c)    Coelenterata                

(d)   Ctenophora

Question 2: Why does the Ctenophora is a minor phylum                                                                      

(a)   It includes small sized animals                               

(b)   It includes only few genera

(c)    It does not include animals of economic importance                    (d)   It was included earlier in cnidaria

Question 3: Which of the following animals has a tetramorphic colony?

(a)   Obelia                             

(b)   Physalia                        

(c)    Porpita                          

(d)   Velella

Question 4: Which of the following animals is without sexual medusae?

(a)   Halistemma                 

(b)   Hydra                            

(c)    Millepora                      

(d)   Velella

Question 5: Which coelenterate is not polymorphic?                                                                             

(a)   Porpita                          

(b)   Vellela                            

(c)    Adamsia                        

(d)   Physalia